1. Field of the Invention
The invention relates to a drive control device for a vehicle and, more particularly, to a technique for, in the vehicle that is able to travel in coasting, that is, in a state where engine braking force is decreased as compared to that in engine braking traveling, achieving both fuel economy and drivability of the vehicle during coasting on an upward-gradient road surface.
2. Description of Related Art
In contrast to engine braking traveling in which a vehicle travels while engine braking is being applied through driven rotation of an engine in a state where a power transmission path between the engine and wheels remains connected, there is conceivably coasting in which a vehicle travels while engine braking force is decreased as compared to that in the engine braking traveling in order to contribute to improvement in fuel economy by extending a travel distance. One example is a device described in Japanese Patent Application Publication No. 2002-227885 (JP 2002-227885 A). The device is configured to, when it is determined that an accelerator pedal has been returned while the vehicle is traveling, the vehicle is caused to start coasting by releasing a clutch provided in a power transmission path between an engine and wheels, thus improving the fuel economy of the vehicle.
Incidentally, the coasting of the vehicle conceivably includes free-run coasting in which the engine is disconnected from the wheels by releasing the clutch in the power transmission path and rotation of the engine is stopped by stopping supply of fuel to the engine and neutral coasting in which the engine is operated by being supplied with fuel in a state where the engine is disconnected from the wheels by releasing the clutch in the power transmission path. However, JP 2002-227885 A does not consider in which coasting the vehicle is caused to travel on the basis of an upward gradient of a road surface at the time when the vehicle starts coasting. Therefore, it is not possible to appropriately cause the vehicle to coast on the basis of the upward gradient of the road surface, and there may occur the following inconvenience.
When the vehicle is coasting on an upward-gradient road surface, accelerating force acts in a direction opposite to a traveling direction due to gravity, so a coasting distance reduces as compared to that when the vehicle coasts on a flat road or a downward-gradient road surface. Therefore, when the vehicle is caused to travel in the free-run coasting at the time when the upward gradient is large, frequent engine stop and restart occur in a relatively short distance, so the drivability of the vehicle decreases. In contrast to this, engine start and restart are not required in the neutral coasting, so deterioration of drivability does not occur irrespective of the coasting distance. However, if the vehicle is caused to travel in the neutral coasting also at the time when the upward gradient is small, that is, when the coasting distance is long, an opportunity to stop the engine reduces, so the fuel economy of the vehicle relatively deteriorates.